Comets, not asteroids, to blame for moon's scarred face

Icy comets – not rocky asteroids – launched a dramatic assault on the Earth and moon around 3.85 billion years ago, a new study of ancient rocks in Greenland suggests. The work suggests much of Earth's water could have been brought to the planet by comets.

"We can see craters on the moon's surface with the naked eye, but nobody actually knew what caused them – was it rocks, was it iron, was it ice?" says Uffe Gråe Jørgensen, an astronomer at the Niels Bohr Institute in Copenhagen, Denmark. "It's exciting to find signs that it was actually ice."

Evidence suggests that the Earth and moon had both formed around 4.5 billion years ago. But almost all the craters on the moon date to a later period, the "Late Heavy Bombardment" 3.8 to 3.9 billion years ago, when around 100 million billion tonnes of rock or ice crashed onto the lunar surface. The Earth would have been pummelled by debris at the same time, although plate tectonics on our restless planet have since erased the scars.

To find out whether asteroids or comets were the main culprits for the bombardment, Jørgensen decided to measure levels of the element iridium in ancient terrestrial rocks. Iridium is rare on the Earth's surface because almost all of it bound to iron and sank into the Earth's core soon after the planet had formed. But iridium is relatively common in comets and meteorites.

Rock or ice

His team calculated the amount of iridium that asteroids would leave on the Earth and moon compared to comets. Because comets have more volatile elements and higher impact speeds due to their more elongated orbits around the sun, they would create giant plumes on impact, allowing more iridium to escape into space than during asteroid impacts.

The team predicted that asteroid bombardment would leave iridium levels of 18,000 and 10,000 parts per trillion in rocks on the Earth and moon respectively, while the same figures for comet bombardment would be about 130 and 10.

Ancient moon rocks returned by NASA's Apollo missions have already confirmed that the lunar iridium levels are 10 parts per trillion or less. To find out the terrestrial value, Jørgensen's team sampled some of the world's oldest rocks from Greenland, aged 3.8 billion years, and asked a Japanese laboratory to assess their iridium levels more accurately than ever before. They contained iridium levels of 150 parts per trillion.

Giant plumes

If so, Jørgensen's team calculates that around 3400 tonnes of icy comet material fell on each square metre of the Earth. About half the comet material would ricochet back into space in giant plumes, leaving behind roughly a billion cubic kilometres of cometary water in total.

That is a similar amount to that in the Earth's oceans today, although it is not clear whether there was already water on the planet due to chemical reactions on the early Earth (see Earth's water brewed at home, not in space).

Michael Mumma, a comet expert at NASA's Goddard Space Flight Center in Maryland who was not involved in the research, says the new report is interesting: "The paper is certain to stimulate lively debate."

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A new study suggests comets gouged out the vast majority of craters on the moon (Image: NASA)